1. Field of the Invention
The present invention relates to an electrostatic chuck for holding a wafer during treatment such as forming a thin film on the wafer or etching the same in a process for producing semiconductor devices according to the plasma CVD method or the plasma etching process and in particular to an electrostatic chuck which is capable of maintaining the temperature of the wafer uniform throughout the processing.
2. Description of the Prior Art
Until now, in the plasma CVD apparatuses and the plasma etching apparatuses, a wafer to be processed has been positioned on a substrate-supporting base under the influence of gravity or mechanically clamped to the base.
In these methods for positioning a wafer in the foregoing apparatuses, for instance, a method for positioning a wafer under the influence of gravity, the temperature of a wafer greatly increases in proportion to an increase in processing energy and the patterns formed on the wafer in the preceeding process are correspondingly damaged because of the low heat transmission rate in a vacuum. Therefore, the wafer cannot be treated at a high processing rate. On the other hand, in the clamping method, the processing precision of a wafer is impaired in the vicinity of the clamping tool. Therefore, the clamping of only a part of the periphery of the wafer is permitted during the processing, the contact pressure between the wafer and a base for cooling is reduced and accordingly the cooling efficiency is almost the same as that for the method in which the wafer is positioned under the influence of gravity. Moreover, the wafer causes mechanical strain even if a gas serving as a heat transmission medium is introduced between the wafer and the cooling base, and thus the uniform distribution of the wafer temperature cannot be ensured.
Thus, there has been a desire for the development of low temperature processing in order to solve the aforementioned problems and to finely and uniformly process the wafer. Under such circumstance, an electrostatic chuck which makes use of electrostatic attractive force has been developed and already put into practical use.
However, the electrostatic chuck which has been put into practical use likewise suffers from a problem of establishment of uniform temperature distribution in a wafer. To solve this problem, for instance, Japanese Patent Application Laid-Open No. 1-251735 discloses an electrostatic chuck having a complicated structure designed so as to introduce a gas between a wafer and the electrostatic chuck while adjusting the pressure of the gas.